1. Field of the Invention
This invention relates to the levitation of magnetic materials by magnetic fields, and more particularly to the suspension of such materials in a state of stable or indifferent equilibrium relative to gravitational attraction. The invention may be incorporated and used for measurement and control in a wide range of instruments such as accelerometers, flow meters, gravity meters, gyroscopes, magnetometers, micrometers, and tilt meters, as well as in nonmeasurement devices such as bearings and other apparatus.
2. Description of the Related Art
There are many applications where it would be desirable to levitate a magnet or a magnetic member. A levitated magnet can be acted on magnetically, and its position can be sensed magnetically. There are significant limitations that must be overcome to levitate a magnet. Magnetostatic or stationary electromagnetic fields will not levitate magnets or magnetic materials unless the field is modified to have a relative magnetic permeability that is less than one at one or more positions. Otherwise, the magnetic material to be suspended will either be expelled from the field or drawn into contact with a magnet providing the field. This phenomenon is referred to as instability. Magnetic fields with stable or indifferent equilibrium can be created by interaction between a permanent magnet and a material having a relative magnetic permeability less than one. The stabilizing magnetic force of such a field is inversely proportional to the relative magnetic permeability.
There are substantial drawbacks to all prior approaches for providing a magnetic permeability that is less than one in order to achieve levitation.
Diamagnetic materials have relative magnetic permeabilities that are lower than one and can be used to provide stable levitation. But the permeabilities are only slightly lower than one and provide low lift force. Large permanent magnets have been used to levitate diamagnetic members. But that is very different from levitating a magnet. One prior reference suggests that the interaction between a magnet and a diamagnetic material could levitate a few microns of the magnetic material. That is not a sufficient volume for use in a practical device. A stronger magnetic field capable of levitating a greater mass of magnetic material would not be attained simply by increasing the mass or quantity of diamagnetic material. Another reference uses a diamagnetic material to provide stability and a second fixed magnet located above a first magnet in order to provide sufficient lift force to levitate the first magnet. Some of the drawbacks of this design are that the second magnet creates substantial magnetic spring constants or forces that distort the motion and limit the stability of the first magnet. The lift magnet also increases size, weight, and cost.
Superconductors have a magnetic permeability of zero and produce higher lift force than diamagnetics. The lift force of the magnetic field produced by the interaction between a magnet and a superconductor is sufficiently large to suspend a magnet in the magnetic field. A permanent magnet of about one gram has been reported suspended above a concave superconducting disk. But applications involving materials in the superconductive state have the limitation of requiring very low temperatures, i.e. around -200.degree. C. or lower. Another approach to levitation is to use a variable electromagnetic field and feedback control to suspend magnetic materials. Drawbacks of electromagnetic devices are that they require power consumption, active control, and increase cost.